Air traffic control involves human staff communicating with the pilots of a plurality of planes, instructing them on routes so as to avoid collisions. Aircraft generally file “flight plans” indicating their routes before flying, and from these, the controllers have some initial information on the likely presence of aircraft, but flight plans are inherently subject to variation (due, for example, to delays in take offs; changes of speed due to head wind or tails wind; and permitted modifications of the course by the pilot). In busy sectors (typically, those close to airports) active control of the aircraft by the controllers is necessary.